1. Field of the Invention
This invention relates generally to facilitating the detection of pipes intended for deployment below ground, so that they may readily be located, and relates more particularly to the development of a means and material for the magnetic detection of plastic pipe.
2. Brief Description of the Prior Art
In many situations it is desirable to locate pipes below the surface of the earth, either to service or repair them or to avoid inadvertent damage when subsequently excavating in the vicinity. Much of the pipe used in construction today, particularly for natural gas lines, is plastic rather than metal.
Plastic pipe is typically polyethylene, but may also be other formulations such as polyvinylchloride (PVC). Plastic pipe has several advantages. It is extremely durable and reliable, relatively lightweight, easy to make, and cheaper than most other materials that may be used. Perhaps most importantly, it essentially inert, such that nothing sent through it will react with it, unlike some other materials.
One problem in using plastic pipe, however, is that once buried it is difficult to find, thus making it difficult to service the pipe and more likely that unintentional damage will result from nearby digging.
Various attempts have been made to solve this problem. Ribbons have been buried above the pipes, so that one could in theory locate the pipe by digging until the ribbon, of a color contrasting to the soil, was found. See, e.g., Allen, U.S. Pat. No. 3,115,861, and Prosser, U.S. Pat. No. 3,282,057. However, since two separate items must be buried, it is difficult to locate the ribbon accurately over the pipe, and the ribbon may "drift" away from the pipe after it is buried, thus making it more difficult to locate the pipe even if the ribbon is found. Also, the ribbon must later be located by digging as there is no way to detect it from above ground. Finally the ribbons may break or get lost in the soil being excavated, particularly where substantial digging is done, such as by heavy equipment, and may thus be lost before they can be located.
Similarly, metal foils have also been buried above pipes, as in Allen, U.S. Pat. No. 3,504,403, and Allen et al., U.S. Pat. No. 3,633,533, the alleged advantage being that these may be detectable from the surface by a metal detector device. Again, however, the problems of accurate placement of the ribbon over the pipe and the possible drift render this solution unsatisfactory.
A variation on this theme is shown in Southworth et al., U.S. Pat. No. 3,568,626, which describes reinforcing a metal foil with some other material, such as nylon, fiberglass or steel, so that the foil is not easily broken, and using shorter portions or stretchable portions so that an equipment operator will hopefully see a portion uncovered by the excavation before the pipe is damaged. The metal foil may contain either magnetic or radioactive material so that once that part of the foil is located by digging, the remainder can be followed from above ground by an appropriate detector. Again, however, the foil must be buried over the pipe, and problems of drift and breakage remain. As with colored ribbons, it is possible that the operator will not see the foil before striking the pipe.
Keene, U.S. Pat. No. 4,573,829 involves a somewhat similar concept, in which a wire is placed inside the pipe. The wire can be detected either by a magnetometer, or if the wire is magnetic, or by applying an R.F. signal to the wire and locating it with an R.F. sensing device. However, just as the ribbons described above require an extra step to bury, an extra step is required here to insert the wire into the pipe during construction of the pipeline. Also, if it is desirable to apply the R.F. signal to the wire, the ends must be located to do so. Finally, it is possible that the wire may break, thus preventing detection by R.F. methods, since an open circuit would result and no signal could be applied in such a case.
There have been other efforts to mark the location of an underground pipe, such as placing markers in the ground or inscribed in the pavement near the location of the pipe. These are generally not accurate enough to solve the problem, and much time can be wasted in locating the pipe even when there is a marker in the general vicinity.
It is felt that none of these methods solve the problem of allowing for a plastic pipe to be buried in a single step, without additional inconvenience or having to bury an additional object, while permitting detection from above ground after burial by ordinary means and without locating a buried ribbon or foil or the ends of a wire.